The technical field of the present invention is the production of β-γ-TiAl alloys in a melting metallurgical process by means of vacuum arc remelting (VAR). In prior-art methods, the raw materials sponge titanium, aluminum as well as alloy elements and master alloys are compacted to form compact bodies which contain the desired alloy components in the correct stoichiometric ratio. If necessary, evaporation losses caused by the subsequent melting process are pre-compensated. The compacts are either molten directly to form so-called ingots by means of plasma melting (PAM) or they are assembled to form consumable electrodes which are then molten to form ingots (VAR). In both cases, materials are produced whose chemical and structural homogeneity is not suitable for technical use and which therefore need to be remolten at least once (see V. Guether: “Microstructure and Defects in γ-TiAl based Vacuum Arc Remelted Ingot Materials”, 3rd Int. Symp. On Structural Intermetallics, September 2001, Jackson Hole Wyo., USA).
DE 101 56 336 A1 discloses a method for the production of alloy ingots which comprises the following method steps:    (I) production of electrodes by mixing and compacting the selected materials in the usual manner;    (ii) remelting the electrodes obtained in (I) at least once in a conventional melting metallurgical process;    (iii) induction melting of the electrodes obtained in (I) or (ii) in a high-frequency coil;    (iv) homogenizing the melt obtained in (iii) in a cold wall induction crucible; and    (v) removing the melt, solidified by cooling, from the cold wall induction crucible used in (iv) in the form of blocks having a freely selectable diameter.
DE 195 81 384 T1 describes intermetallic TiAl compounds and methods for the production thereof, with the alloy being produced by heat treatment at a temperature in the range of 1300° C. to 1400° C. of an alloy having a Ti-concentration of 42 to 48 atomic %, an Al-concentration of 44 to 47 atomic %, an Nb-concentration of 6 to 10 atomic % and a Cr-concentration of 1 to 3 atomic %.
DE 196 31 583 A1 discloses a method for the production of a TiAl—Nb product of an alloy in which an alloy electrode is produced from the alloy components in a first step. The alloy electrode is formed by compacting and/or sintering the alloy components to form the electrode. The electrode is molten by an induction coil.
JP 02277736 A discloses a heat-resistant TiAl base alloy in which specific amounts of V and Cr are added to an intermetallic TiAl-compound to improve the heat-resistance and ductility thereof.
Finally, DE 1 179 006 A discloses ternary or higher titanium aluminum alloys containing elements which stabilize the α- and β-phase of the titanium.
The process of vacuum arc remelting using a consumable electrode is the usual method for remelting as the plasma melting furnaces are usually not designed for supplying starting materials in the form of compact ingots. In the example of conventional two-phase γ-TiAl base alloys comprising lamellar colonies of the α2-TiAl3 phase and the γ-TiAl phase, remelting in the vacuum arc remelting furnace (VAR furnace) occurs without any difficulties so that the desired result is obtained (see V. Guether: Status and Prospects of γ-TiAl Ingot Production”; Int. Symp. On Gamma Titanium Aluminides 2004, ed. H. Clemens, Y.-W. Kim and A. H. Rosenberger, San Diego, TMS 2004).
A new generation of γ-TiAl high-performance materials such as the so-called TNM®-alloys of the applicant possesses a structure which is different from conventional TiAl alloys. In particular by reducing the aluminum content to usually 40 at. % to 45.5 at. % and by adding β-stabilizing elements such as Cr, Cu, Hf, Mn, Mo, Nb, V, Ta and Zr, a primary solidification path is obtained in the β-Ti-phase. The result is a very fine structure which contains lamellar α2/γ colonies as well as globular β grains and globular γ grains, sometimes even globular α2 grains. Materials having such structures possess decisive advantages in terms of their thermo-mechanical properties and their processibility by means of forming technologies (see H Clemens: “Design of Novel β-Solidifying TiAl Alloys with Adjustable β/B2-Phase Fraction and Excellent Hot-Workability”, Advanced Engineering Materials 2008, 10, No. 8, p. 707-713). As already mentioned at the outset, such alloys are hereinafter referred to as β-γ-TiAl base alloys.
The drawback is that when electrodes of this material are remolten again in the VAR furnace, cracks are formed which often cause components of the consumable alloy electrode to chip off the electrode in the initial melting zone. These chippings fall into the molten pool where they are not completely remolten again. This causes structural defects in the ingot, with the result that the ingot material is no longer suitable for use. Under these conditions, remelting in the VAR furnace is no longer possible in a technically reproducible manner.
The undesirable chipping behavior is supposed to be caused by massive phase shifts in the temperature range between the eutectoid temperature and the phase limit temperature to the β single phase region. In particular in the event of phase shifts, the different linear expansion coefficients of the various phase components cause sudden changes of the integral linear heat expansion coefficient of the alloy, which results in internal stresses that exceed the stability of the material in the given temperature range.
Corresponding dilatometer measurements in a TNM®-B1-alloy (Ti-43.5 AL-4.0 Nb-1.0 Mo-0.1 B at. %) show that the linear expansion coefficient of a corresponding alloy sample is more than multiplied in the temperature range between 1000° C. and 1200° C., in other words it increases from 9×10−6 to 40×10−6K−1. This behavior is shown in the attached FIG. 4 where the curve A represents the linear expansion coefficient of this alloy. The line R represents the heating rate of the sample.
During VAR melting, a temperature field from melting temperature (approx. 1570° C.) at the lower side of the electrode to almost ambient temperature at the electrode suspension extends through the material relative to the length of the consumable electrode. Near the melt front, the critical temperature range of between 1000 and 1200° C. is reached. In this zone, the relatively poor ductility of the intermetallic material causes cracks to form in this zone as a result of the stresses occurring there, which in turn cause non-molten pieces to chip off the electrode as described above.